1. Field of the Art
The present invention relates to a p-phenylene sulfide copolymer. More particularly, the invention relates to a crystalline p-phenylene sulfide block copolymer comprising a block of p-phenylene sulfide recurring units ##STR5## in the molecular chain.
2. Prior Art
Concerning p-phenylene sulfide polymers, there have been numerous reports on p-phenylene sulfide homopolymers (as disclosed in the specifications of Japanese Patent Publications Nos. 12240/1977 and 3368/1970 and Japanese Patent Laid-Open No. 22926/1984). Also, some reports can be found on p-phenylene sulfide random copolymers (as described, for example, in the specification of U.S. Pat. No. 3,869,434).
The p-phenylene sulfide homopolymers have been used as heat-resistant thermoplastic resins mainly in injection molding processes since the highly crystalline p-phenylene sulfide homopolymers can be used at a temperature as high as nearly their crystalline melting point (about 285.degree. C.) when they are highly crystallized. However, these polymers have been accompanied by the problems of excessively high crystallization rate in the melt process and ready formation of rough spherulites. That is, when films are to be formed from them by an inflation method, they are crystallized and solidifed prior to sufficient inflation, whereby it is difficult to form intended stretched and oriented films. In extrusion molding by means of a T-die to form a sheet, the crystallization and solidifying occur prior to the winding of the sheet around a wind-up roll, whereby it is difficult to obtain a smooth sheet having a uniform thickness. In melt extrusion to form pipes, rough spherulites are formed prior to the quench to make it difficult to obtain the tough extrusion moldings. In melt coating of electric wires, rough spherulites are formed in the coating film to make it difficult to obtain tough coating films. In the production of fibers by melt spinning process, the crystallization and solidifying proceed in the course of the melt spinning operation to make sufficient stretch and orientation impossible, and, therefore, tough fibers cannot easily be obtained.
While the p-phenylene sulfide random copolymers, which are generally non-crystalline, have a characteristic feature of being melt-processed quite easily, since they are not crystallized or solidified in the course of the melt spinning operation, they are problematic in that their heat resistance is extremely poor due to the non-crystallizability.
Printed circuit boards composed of an insulating base and a metal layer of a circuit pattern formed on the surface thereof have been used widely in the field of electronic appliances.
As the insulating materials for the printed circuit boards, composites of thermosetting resins, such as epoxy, phenolic and unsaturated polyester resins, with fibrous reinforcing materials, such as glass fibers, synthetic fibers and paper, have been mainly used. However, these materials are problematic in that a long time is necessary for recovery of the solvent and curing of the resin and in that they have a high hygroscopicity and only a poor resistance to CAF (conductive anodic fiber growth).
Recently, attempts were made to use a composite of poly-p-phenylene sulfide which is a thermoplastic resin and a fibrous reinforcing material for the production of insulating bases for printed circuit boards (as described in the specifications of Japanese Patent Laid-Open Nos. 96588/1982 and 3991/1984). However, the insulating base comprising the poly-p-phenylene sulfide has insufficient adhesion to the metal layer, and, therefore, the metal layer is easily peeled off.
Electronic components such as IC, transistors, diodes and capacitors have been sealed with or encapsulated within a synthetic resin or ceramic substance for the purposes of preventing changes in the properties due to the external atmosphere, preventing deformation, and maintaining the electrical insulating property.
The sealing resins used heretofore include thermosetting resins, particularly, epoxy and silicone resins. However, these resins have the following defects: (1) the molding time is prolonged, since a long time is necessary for the thermosetting, (2) a long post-curing time is required, (3) as the molding shot number is increased, contamination of the mold accumulates, (4) the resin is easily deteriorated during storage and (5) unnecessary portions like runner gates of the moldings cannot be reused.
For overcoming the above mentioned drawbacks, processes wherein poly-p-phenylene sulfide (a thermoplastic resin) is used have been proposed (as described, for example, in the specifications of Japanese Patent Publication No. 2790/1981 and Japanese Patent Laid-Open Nos. 22363/1978, 81957/1981, 20910/1984 and 20911/1984).
When poly-p-phenylene sulfide is used, the sealing or encapsulation is conducted ordinarily by a melt molding process. In this process, the crystallization proceeds rapidly to form rough spherulites in the step of solidifying the molten resin. Therefore, a marked molding shrinkage occurs in the resin layer, particularly around the spherulites, to form cracks in the resin layer, to cut or to deform the bonding wire, and to form a gap between the lead frame or bonding wire and the resin layer. As a result, a problem arises in the resulting electronic parts in the water penetrates thereinto through the interface between the resin layer and the lead frame or bonding wire to cause deterioration of the quality of the electronic parts particularly at a high temperature in a highly humidity atmosphere. To solve these problems, processes wherein inorganic fillers or various additives are used have been proposed. However, the problems cannot be solved essentially unless the properties of the base resin are altered.